The present invention is directed to a warm-up circuit by means of which the hydraulic fluid in the control system which supplies operating pressure to the hydraulically actuated pilots of a directional control valve of a primary hydraulic system may be warmed up to a normal operating temperature during the warm-up period for the primary system.
The variation of the viscosity of hydraulic fluids with temperature is such that noticeable differences in the response of hydraulically actuated devices will be observed when comparing the response of the system with the fluid at ambient or room temperature to the response of the system when the hydraulic fluid has reached a normal operating temperature typically 110.degree. F. or higher. It is thus conventional practice where the hydraulic system has been shut down over night to warm up the fluid in the primary system by operating the main system pump for periods of 20 minutes to 1 hour, depending upon the systems, before placing the system in operation. During this warm up period, the directional control valve of the primary system is maintained in its centered position, in which the pressure port of the valve is internally connected to the tank or return port so that fluid from the pump is continuously circulated in a closed loop system or via the tank is an open loop system. The flow of fluid through the hydraulic lines and the pump will raise the temperature of fluid to its normal operating temperature.
Where the directional control valve of the primary system is pilot operated, a problem arises in that the four way directional control valve employed to control the pressure to the pilots of the valve of the primary system is a float center type valve in which, when the valve is in its centered position, both control ports are connected to the tank or return port and the pressure port is blocked. It is thus not possible or practical to attempt to circulate hydraulic fluid in the pilot control circuit to warm this fluid up, and as a result there will normally be a sluggishness in the shifting of the primary system valve until the fluid in the pilot circuit reaches a normal operating temperature. While the slight delay in actuation of the primary system valve due to cold hydraulic fluid in the pilot operating circuit is of little consequence in some applications, it is unacceptable in those applications where the primary hydraulic system is employed to precisely position a tool or work piece because a slight delay in shifting the primary control valve will result in an undesired over travel of the tool or part being positioned.
The present invention is especially directed to a solution to the foregoing problem.